Menstruation menstruation occurs after ovulation but prior to degeneration of the follicle involves the breakdown of the endometrium is stimulated by hcg is triggered by an lh surge

The answer would be: involves the breakdown of the endometrium

To preparing for bearing a child, the endometrium cells will replicate and makes a layer of cells for the egg implantation. Menstruation happens when the eggs are not fertilized. This will cause the lining of the endometrium breakdown, resulting in tissue and blood flowing from the uterus into the vagina. 

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